[daip] Error in parallactic angle

[Eric Greisen]

On 01/08/2017 11:32 AM, Andy Biggs wrote:
> Hi there,
>
> I'm analysing VLA data from two monitoring campaigns that were observed
> over the same 3-month period. One used 3C119 as the PPA calibrator and
> the other 3C48. 3C119 is far superior as it's unresolved in polarized
> flux and doesn't pass as close to the zenith. If I compare the R-L phase
> corrections, there is a very interesting systematic offset between the
> two within an hour or two of transit - see attached plot.
>
> I showed this to Rick and he suggested an error in the assumed
> parallactic angle due to misalignment of the antenna pads and, sure
> enough, the functional form of that agrees well with what I see (blue
> line on plot). However, to match the data I have to assume an offset of
> ~10' which is about an order of magnitude too big. Ken Sowinski says
> that a few pads were erroneously built parallel to local gravity as
> opposed to the centre of the array and that an error of ~10' would be
> possible for a misaligned pad at the end of an arm, but repeating the
> analysis deleting each antenna one by one makes no difference.
>
> Although very unlikely, I am wondering therefore if it might be possible
> that AIPS is making an error with the source or array position when
> correcting for the parallactic angle. This would be a very neat
> explanation, particularly as it's not clear to me how random
> pad-alignment errors would lead to a systematic offset - I would expect
> them to all cancel out.
>
> Might such a thing be possible? Note that the 3C48 R-L corrections were
> derived from Q and U fluxes measured from maps made using IMAGR.

The current code recognizes the old VLA as being all at the same array 
longitude and latitude but does not recognize the EVLA in the same way.
Thus each antenna will have its own parallactic angle changing 
systematically across the array.  Could that be the explanation?  It 
would be a bit uncertain how to change this since the array name is not 
in the COMMON that the PARANG routine uses and the JVLA is now 0,0,0 
like most other arrays.

Eric